When I started growing 5 years ago I bought a 50 Liter bag. I loose a few pellets every grow, but I have not bought any more (yet) in the kast 5 years.
I soak used pellets for a day to leach the salts, then rinse them a handful at a time to get out the plant matter, and re-use them. Takes some time, but no problems with subsequent grows.
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